Purpose: To compare the precision and consistency of measuring Linear Accelerator Gantry Angles between an Electronic Portal Imaging Device (EPID) and a Spirit Level.

Methods:A cubical phantom was created by placing radio opaque markers therein at precise positions. The phantom was placed on the treatment table and aligned with the lasers in the treatment room. Using the Linear Accelerators EPID, images of the phantom were taken at the four primary gantry angles 0. 90. 180 and 270 degrees. Over a period of 1 month, 25 images were acquired at each primary gantry angle. The images of the phantoms were analyzed to calculate the gantry angle using a macro created with Image J (NIH). In the same time period, 13 gantry angle measurements were made using a spirit level at the four primary angles.

Results:The average and standard deviations of gantry angles for the EPID based method at the four primary gantry angles 0, 90, 180 and 270 were -0.1(0.011), 90.0(0.011), 180.2(0.015) and 270.0(0.006) degrees respectively. In comparison, the spirit level based measurements yielded average gantry angles of 0.2 (0.03), 90.2 (0.04), 180.1(0.01) and 269.9(0.05). Results of each method similar and are within +/- 0.2 degrees of the intended angle. There is however a noteworthy difference in standard deviations between each method.

Conclusion:The precision of each method yielded results within the linac specifications. Improvements to the materials and construction of the phantom could improve gantry angle accuracy by reducing positioning errors in the markers. More data needs to be acquired to determine which method is more accurate. However, the EPID gantry angle measurements strongly suggest an advantage over the spirit level results with more reproducible results as demonstrated by smaller deviations from the mean.